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          锁
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        <h2 id="概念">概念</h2>
<p>锁：在执行多线程时，用于强行限制资源访问的同步机制，即用于并发控制中保证对互斥要求的满足。</p>
<p>锁都是为了互斥（Mutual exclusion，缩写 Mutex）。即防止两条线程同时对同一公共资源（如全局变量）进行读写的机制。</p>
<h3 id="分类">分类</h3>
<p>锁从实现上分两种：</p>
<ul>
<li>自旋锁。效率高、安全性不足、占用CPU资源大。</li>
<li>非自旋锁。安全性突出、占用CPU资源小，但休眠、唤醒过程要消耗CPU资源。</li>
</ul>
<h3 id="自旋转锁">自旋转锁</h3>
<p>忙等待的，会一直在那空转（循环），直到使用锁的一方释放。自旋锁不会让线程状态发生切换，一直处于用户态，即线程一直都是active的，不会让线程进入阻塞/休眠，减少了不必要的上下文切换，执行速度快。</p>
<p>以下情况选用自旋锁：</p>
<ul>
<li>预计线程等待锁的时间很短；</li>
<li>加锁代码（临界区）经常被调用，但竞争情况发生概率很小，对安全性要求不高；</li>
<li>CPU资源不紧张或多核处理器</li>
</ul>
<h3 id="非自旋锁">非自旋锁</h3>
<p>非忙等待的，操作内核，将自己的状态改为阻塞挂起来，从待执行队列中移出，等待其他线程唤醒。在获取不到锁的时候会进入阻塞状态，从而进入内核态，当获得锁的时候需要从内核态恢复，需要线程上下文切换，影响锁的性能。</p>
<p>以下情况使用非自旋锁：</p>
<ul>
<li>预计线程等待锁的时间比较长；</li>
<li>单核处理器；</li>
<li>临界区有IO操作；</li>
<li>临界区代码复杂度、循环量大</li>
<li>临界区竞争非常激烈，对安全性要求高</li>
</ul>
<h3 id="阻塞与休眠">阻塞与休眠</h3>
<ul>
<li>阻塞：等待一个中断事件的到来</li>
<li>休眠：等待一个超时事件的到来</li>
</ul>
<h2 id="ios中的锁">iOS中的锁</h2>
<p>互斥锁：</p>
<ul>
<li>pthread_mutex_t</li>
<li>NSLock、NSConditionLock（封装了pthread_mutex_t，attr = 普通）</li>
<li>NSDistributedLock（封装了pthread_mutex_t，attr = 递归）
<ul>
<li>引用计数表的数据结构中使用到，对一张表的多个部分进行同时操作。</li>
</ul></li>
<li>NSCondition（封装了pthread_mutex_t和pthread_cond_t）</li>
<li><span class="citation" data-cites="synchronized">@synchronized</span></li>
</ul>
<p>递归锁（基于互斥锁）：</p>
<ul>
<li>NSRecursiveLock</li>
</ul>
<p>自旋锁：</p>
<ul>
<li>OSSpinLock</li>
</ul>
<h4 id="nslock">NSLock</h4>
<p>普通的互斥锁。通过阻塞线程实现。</p>
<h4 id="nsconditionlock">NSConditionLock</h4>
<p>条件锁。比NSLock多了个<code>NSInteger condition</code>作为相等的条件。与<code>condition</code>相等则加锁。</p>
<h4 id="nsrecursivelock">NSRecursiveLock</h4>
<p>递归锁。与NSLock类似，但可以在同一线程重复加锁而不死锁。实现递归过程原子性。</p>
<h3 id="osspinlock">OSSpinLock</h3>
<p>自旋锁。</p>
<h3 id="os_unfair_lock">os_unfair_lock</h3>
<p>用于替代OSSpinLock，解决了优先级反转的问题，但其本质是互斥锁。atomic内部也使用该锁。</p>
<h4 id="nscondition">NSCondition</h4>
<p>协调线程间的顺序执行。wait-signal。先执行一部分任务，然后跳转到其他地方执行，完了以后再回来。</p>
<h3 id="synchronizeobject"><span class="citation" data-cites="synchronize">@synchronize</span>(object)</h3>
<p>通过判断传入的对象是否相同，才满足互斥。</p>
<h3 id="dispatch_semaphore">dispatch_semaphore</h3>
<p>信号量。限制有限数量的资源使用。</p>
<h3 id="pthread_mutex">pthread_mutex</h3>
<p>互斥锁。</p>
<h2 id="实现多线程多读单写">实现多线程多读单写</h2>
<p>实现方案：</p>
<ul>
<li>pthread_rwlock，读写锁</li>
<li>dispatch_barrier_async，异步栅栏调用
<ul>
<li>需要在一个自己创建的并发队列中执行屏障。</li>
</ul></li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">//手动创建一个并发队列</span><br><span class="line">dispatch_queue_t queue = dispatch_queue_create(&quot;rw_queue&quot;, DISPATCH_QUEUE_CONCURRENT);</span><br><span class="line">dispatch_async(queue, ^&#123; // 普通异步</span><br><span class="line">    /*</span><br><span class="line">     读操作代码</span><br><span class="line">     */</span><br><span class="line">&#125;);</span><br><span class="line">dispatch_barrier_async(queue, ^&#123; // 屏障异步</span><br><span class="line">    /*</span><br><span class="line">     写操作代码</span><br><span class="line">     */</span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>
<p><img src="https://gitee.com/bqlin/image-land/raw/master/7POMTw.jpg" /></p>
<h2 id="参考">参考</h2>
<ul>
<li><a target="_blank" rel="noopener" href="https://juejin.cn/post/6844903993563414535">iOS-关于锁的总结 - 掘金</a></li>
<li><a target="_blank" rel="noopener" href="https://www.jianshu.com/p/9447e55fa79d">IOS - 自旋锁和atomic - 简书</a></li>
<li><a target="_blank" rel="noopener" href="https://ityongzhen.github.io/%E5%85%B3%E4%BA%8EiOS%E4%B8%AD%E7%9A%8413%E7%A7%8D%E5%8A%A0%E9%94%81%E6%96%B9%E6%A1%88.html/">关于iOS中的13种加锁方案 | 殷永振</a></li>
<li><a target="_blank" rel="noopener" href="https://zhuanlan.zhihu.com/p/335166736">iOS进阶-细数iOS中的锁 - 知乎</a></li>
<li><a target="_blank" rel="noopener" href="https://juejin.cn/post/6965770220921159694">如何保证iOS的多线程安全 - 掘金</a></li>
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